Plant growth regulators such as auxins, cytokinins, and gibberellins are useful for influencing a range of plant developmental processes including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction, fruit size and quality, as well as leaf and fruit senescence.
For example, there are known growth stimulators based on naturally occurring and synthetic auxins, such as indoleacetic acid and naphthaleneacetic acid, which induce stem elongation and promote root formation. Other synthetic auxins include 4-chloro-2-methylphenoxyacetic acid (MCPA); 2,4-chlorophenoxyacetic acid (2,4D); 2,4,5-trichlorophenoxyacetic acid (2,4,5-T); 2-(4-chloro-2-methyl-phenoxy) propionic acid (CMPP); 4-(2,4-dichlorophenoxy) butyric acid (2,4-DB); 2,4,5-trichlorobenzoic acid (TBA); and 3,5-dichloro-2-methoxybenzoic acid (dicamba), for example. All the above acids are active in the form of their salts and esters, such as their sodium, potassium, ammonium, dimethylamine and ethanolamine salts, and their lower alkyl esters. Many of these synthetic auxins are being used commercially as effective herbicides and some of them are known to adversely affect morphogenesis of treated plants. Some auxins, however, such as 3-indolebutyric acid (3-IBA), have been shown to exhibit high instability in aqueous systems.
Preparations based on cytokinins, such as 6-furfurylamino purine and 6-benzlyamino purine, are also known to be growth stimulators. However, cytokinins-based preparations which have a decisive influence in the stimulation of cell division seldom produce a desirable effect in the absence of auxins. While the mechanism by which cytokinins affect the growth cycle of plants is far from being understood, it is apparent that they affect leaf growth and prevent aging in certain plants. While the action of cytokinins on the growth of cultivated plants has been extensively studied, these plant hormones did not find wide application in plant raising since they must be applied at specific concentrations in parts per million. These critical rates of application render cytokinins-based preparations highly impractical in an agricultural environment.
Of all the known stimulators, the most widely used is a series of natural plant hormones generically named “gibberellins”. The gibberellins are used for the acceleration or regulation of various stages of plant development, particularly growth, efflorescence, germination and parthenocarpy of higher plants. A series of related compounds identified as gibberellin A, through A44 has been obtained by microbiological synthesis and the various compounds isolated from culture broth of Gibberella fujikuroi and from various plants including certain beans. The main component of the gibberellins used in practice is gibberellin A3, otherwise known as gibberellic acid.
While gibberellins are highly effective as plant growth promoting or regulating substances, their use is greatly limited by their expense and insufficient effectiveness at low concentrations. As a result, considerable research has concentrated on efforts to find synergistic agents which can be used to enhance the activity of gibberellins. One such synergistic agent for use with the gibberellins that have been discovered and put to practical use is described in U.S. Pat. No. 4,507,144 to Aloni. This patent discloses a composition consisting of the auxin naphthaleneacetic acid (NAA) and gibberellic acid (GA3) used for application to growing plants in order to increase the fiber content of the plants. However, the patented composition does not find wide application in plants other than those disclosed as being used as a source of commercial fibers and show little efficiency in stimulating growth, flowering and fructification of horticultural crops. Moreover, since the disclosed composition is applied to the plant as an aqueous spray, appreciably quantities of the composition flows down onto the soil and is not absorbed and assimilated in a systemic manner by the plant. Another disadvantage of the particular aqueous composition described by Aloni, which is especially specific when a spraying technique is employed, is the reduction of crop quality caused by the impossibility of attaining equally uniform application of the aqueous spray to various parts of the treated plant. A further disadvantage resides in the relatively high water requirements for the preparation of the reference compositions, the consumption of water being up to 800 liters per hectare.
Gibberellin solution formulations are disadvantageous in several respects. The solutions, such as those of GA4+7 in propylene glycol, are less concentrated due to low solubility of actives, and have limited stability. Of the currently used solvents, isopropyl alcohol and methyl alcohol offer severe disadvantages such as flammability and toxicity, which lead to restrictions in manufacturing, packaging, labeling, transportation, and warehousing of such solutions. THFA, used in some of the formulations, is considered corrosive to the eye and skin. Moreover, low solubility of gibberellins in propylene glycol does not permit preparation of high potency solution formulations. These low strength solution formulations also require larger packaging, more storage space, and higher associated transportation, warehousing, and container disposal costs. Due to very low solubility and undesirable hydrolysis, it has not been possible to formulate gibberellins in aqueous systems.
Some plant growth regulators may be prepared as water-dispersible granules. To prepare the water-dispersible granules for spray application, they are dispersed in water and form a suspension upon agitation. Many different water-dispersible granular formulations are known for agricultural chemicals. For example, EP 0 252 897 and U.S. Pat. No. 4,936,901 disclose encapsulated plant growth regulators in water dispersible granular formulations; and U.S. Pat. No. 5,622,658 discloses an extrudable composition for preparing water-dispersible granules. U.S. Pat. No. 6,984,609 discloses a water-soluble granular composition including at least 40% of at least one gibberellin as plant growth regulator, at least one binder, at least one disaccharide and at least one surfactant.
Water-dispersible granules usually have no greater than eight percent moisture content, and form suspensions when added to aqueous solutions. The resulting suspension must be agitated for a period of time in order to fully disperse it. Agitation or by-pass recirculation of the tank-mix must also be maintained during application. The quality of water-dispersible granules is highly process- and active-ingredient-dependent and can result in low yield recoveries, poor attrition resistance leading to dust potential, high manufacturing cost and poor dispersion. Generally, sprays of dissolved water-dispersible granular formulations leave undesirable insoluble residues on the treated foliage and fruit.
For plant growth regulators such as gibberellins to be efficacious, the active ingredient must solubilize in the tank-mixes prior to application. Otherwise, product efficacy will be severely affected. When water-dispersible granules are used, the grower often may not be able to realize if he had achieved the total solubility of the active ingredient in the spray solutions. In addition, water-dispersible granules can become hardened over time and thus result in poor dispersibility and solubility of the active ingredient. Dust and caking may be problems with certain water-dispersible granules and powder formulations.
Attempts have been made to combine various plant growth regulators into a single formulation. U.S. Pat. No. 5,188,655, for example, discloses a mixture of gibberellins, the heteroauxin indole-3-acetic acid and the cytokinin 6-(4-hydroxy-3-methyl-2-trans-betenylamino)purine in definite proportions. In addition to the aforementioned problems with plant growth regulating granules, compounding gibberellins with other certain other plant growth regulators, however, has been shown in particular to increase granule instability and reduce solubility. Therefore, the need exists for plant growth regulator formulations that provide high potency and rapid solubility and avoid the problems associated with conventional formulations.